In order to achieve a low carbon society, distributed electric power sources such as photo voltage generation and wind farms are becoming essential. In addition, battery systems have been implemented so as to effectively use electric power generated by distributed electric power sources and rigid power sources (for example, nuclear power plants) that emit less carbon and that stably generate electric power.
In particular, since renewable electric power sources are subject to fluctuations of output electric power that depends on weather conditions, it is preferable that they be used together with a battery system so as to relatively stably supply desired electric power to the users.
An in-house electric power generation system provided with both a distributed electric power source such as a distributed electric power source or a fuel battery and a battery is described for example in Patent Literature 1.
To accelerate the promotion of the foregoing distributed electric power sources and battery systems, it is preferred that the users be able to enjoy high economic effects when they use electric power. For example, it is preferable that the power rates of the electric grid be reduced by prioritizing the supply of electric power generated by a renewable electric power source or electric power charged to a battery system to the user's load (electric equipment). In addition, it is preferable that surplus electric power that is not used for the user's load be sold to an electric power company. However, some users prioritize the use of electric power generated from natural energy (green electric power), not economic effects. Thus, it is preferable that a renewable electric power source and a battery system be operable in an operation mode that is optimum for each user.
On the other hand, renewable electric power sources and battery systems are still expensive. Thus, the user may not implement both a renewable electric power source and a battery system at the same time. Thus, it is preferable that a renewable electric power source and a battery system be independently operable in association with the electric grid.